The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1). In principle, any non-magnetic material could be used for layer 14 but some are more efficient than others, as will be discussed in greater detail below.
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer such as lead 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8–20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible. In contrast, in a CPP device, the total transverse resistance of all layers, other than the free layer, should be as low as possible so that resistance changes in the free layer can dominate.
A related device to the CPP GMR described above is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ cell in magnetic RAMs is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
Currently, all CIP devices use 8 Å of Ru as their choice of antiferromagnetic coupling material with an AP1/AP2 thickness in the range of 10–30 Å. In the CIP case, the AP1 and AP2 thickness cannot be increased since it will reduce the CIP GMR. Although Rh or Ru4 are known to increase the antiferromagnetic coupling strength 2–4 times relative to Ru8, a much higher annealing field (over 30 kOe) is needed. It is therefore not practical to use Rh or Ru4 for CIP structures.
Currently, all prior art CPP devices continue to use the same structure that was developed for CIP devices (Ru8 with AP1/AP2 in the range of 20–40 Å of CoFe) since it has been reasonable to assume that the same very high field during annealing would be needed.
The present invention discloses how Rh and Ru4 may be used, thereby increasing the CPP GMR, without the need of the afore-mentioned very high annealing fields
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,493,195, Hayashi et al describe using Rh between pinned layers. In U.S. Pat. No. 6,462,541, Wang et al disclose Rh as an antiferromagnetic coupling layer. Grill (U.S. Pat. No. 6,456,469) teaches that any suitable non-magnetic material can be used between the pinned layers, but that Ru is preferred. Many other patents also teach that many materials are suitable. U.S. Pat. No. 6,430,015 (Ju et al)—a Headway patent—shows that Rh, Cr, and Ir can be substituted for Ru. Ueno et al. (U.S. Pat. No. 6,340,533) teaches using Cr, Rh, Ir, and alloys of Ru. Finally, in U.S. Pat. No. 6,338,899, Fukuzawa et al. show that Ru, Rh, Cr, and Ir, or the like, can be used between the pinned layers.